Secondary distribution line overvoltage

What is the probable cause of overvoltage in three phase secondary distribution line? Is it Possible to have an unbalance voltage in secondary distribution line? How do we solve this?Situation: 1-disconnecting switch of a three phase transformer bank has been open in Primary distribution line due to dust and foreign matters and no fault occurs in the line. Wye-delta connection.

Situation: 1-disconnecting switch of a three phase transformer bank has been open in Primary distribution line due to dust and foreign matters and no fault occurs in the line. Wye-delta connection.

Do i interpret correctly that you are opening just one phase of the feed to a three phase wye transformer primary ?

So long as the neutral connection is solid, the voltage across and 120 degree phase angle between other two phases will be maintained and the secondary can continue to operate as an open delta.
But if the neutral connection is not solid, what happens to the phase angles?

Have you taken measurements? How much overvoltage are you seeing? Are secondary phase to phase voltages balanced?

yes sir, we open 1-phase only of the feeder and we didn't take any measurement because we have to re-energize the line asap so we only assume that the line were unbalanced but we are conducting investigation for our report the possible cause of the over voltage because there some damages. Neutral are connected properly though.

so secondary phase to phase voltages are balanced, each is 1/√3 unit X turns ratio,
and the 120 degree phase relationship is intact.

Now remove the neutral connection between transformer and its source...
Phases An and Cn no longer are 120 degrees out because
On primary side , voltages across windings A and C are now in phase not out by 120 degrees, because the neutral is missing so can't pull their midpoint over to center of phasor triangle.
They are just in series .
So each sees half Vac, or 0.5 unit. No problem yet, for 0.5 is less than 1/√3
BUT
On secondary side, phase B is connected between A and C.

I'd look for an open in the neutral wire that connects those transformer primaries back to the next transformer or bus upstream.

I guess i'd summarize this so:
Draw two unit vectors emanating from a common point at 120 degrees to each other.
What is distance between their outboard ends?
Now rotate them so they align.
Their ends moved further apart by ratio 2/√3. That's 15% more distance.

So --- will your phase AC voltage, if you disconnect phase B with an open present( but not noticed because things were balanced ) in the neutral wire somewhere back on the line, not increase by 15% ?

If neutral current is returning via earth, you'll get overvoltage less than 15%.

But - this is just a thought experiment from probably a long distance away.

Probably it's something else . It's very late here and i probably made a mistake - catch you in the morning..

Good luck guys !
That was my best guess as to "probable cause".
I saw something similar once when we [STRIKE]opened[/STRIKE] disconnected the neutral wire to a voltage regulator and didn't know a phase fuseholder was open.

Are primary side transformer neutrals perhaps intentionally not connected to the high side(primary)distribution system neutral?

Sometimes with a small wye-delta transformer bank it is necessary to NOT make that connection, in order to prevent third harmonics from circulating around the delta secondary .
[A small transformer bank driven by a much bigger one in four wire wye-wye-delta can try to absorb all the third harmonic current and overheat. By not tying the small transformer's neutral to the large one's neutral , you block third harmonic current. So, somebody might have wired them that way on purpose.]

I just assumed that "properly" means 'neutrals are connected to one another AND to primary side neutral' .
But you could have meant "connected to each other but not to primary neutral, as designed."
That would explain your symptom.